1. Field of the Invention
The present invention relates to a method for producing a light emitting device, more particularly to a method for forming a contact layer of indium gallium arsenic-phosphorus (InGaAsP) in a light emitting device.
2. Description of the Related Art
A light emitting device such as a semiconductor laser diode (LD) or light emitting diode (LED) has come into increasing use as a light source in optical communication or data processing systems. When so used, the light emitting device is required to have a long life.
A light emitting device usually uses a contact layer of InGaAsP into which impurities can be incorporated or dopants (such as Zn, and Cd) at a high concentration, for example, an order of 10.sup.19 /cm.sup.3. When the concentration of the impurities is reduced, the contact resistance (Rs) between the contact layer and an electrode formed on the contact layer is increased. Therefore, the above-mentioned high concentration of impurities or dopants is required for a low contact resistance. Such impurities are easily highly incorporated into InGaAsP, but not into InP.
However, when a InGaAsP contact layer is formed on semiconductor layers, very small particles of indium are often left on the surface of the InGaAsP contact layer. The reason for this is that after the formation of an uppermost InGaAsP contact layer by the liquid phase epitaxial growth, some melt is left on the contact layer, which is an indium rich melt (90%.about.), especially, due to the wave state of the surface of the InGaAsP contact layer, some particles of indium, which is present in large amounts in InGaAsP, are left. The small indium particles detract from the formation of electrodes on the InGaAsP contact layer or lead to release of the electrodes, these reducing the lifetime of the semiconductor light emitting device.
Japanese Unexamined Patent Publication No. 55-55522 discloses a liquid phase epitaxial process wherein, after forming an upper semiconductor layer, an additional layer of InP or InGaAsP is formed thereon to remove indium left thereon, and, after cooling, the additional layer of InP or InGaAsP is etched by a well known etchant to expose the upper semiconductor layer. However, the process of the prior art does not sufficiently eliminate the indium particles on the upper semiconductor layer of InGaAsP.